MPEG-2 video is used to support both high-definition television (HDTV) and standard-definition television (SDTV). The video frames in HDTV are of higher resolution than those used in present NTSC signals while the video frames of SDTV have approximately the same resolution per frame as the existing analog NTSC standard. Because HDTV provides higher resolution than SDTV, more data is generally required to represent an HDTV frame than is required to represent a corresponding SDTV frame. Accordingly, it is possible to transmit multiple SDTV programs in the same bandwidth required to support a single HDTV program.
For SDTV, MPEG-2 Main Profile at Main Level (MP@ML) specifies various requirements for an MPEG compliant standard definition television signal and associated decoding equipment. MP@ML allows pictures as large as 720×576×1.5 pels (4:2:0) for a total of 622,080 pels per picture. For HDTV, MPEG-2 Main Profile at High Level (MP@HL) allows for pictures as large as 1920×1080×1.5 pels (4:2:0) for a total of 3,110,400 pels per picture.
MPEG-2 video coding employs a motion-compensated discrete cosine transform (DCT) algorithm. The DCT exploits spatial redundancy, and motion compensation exploits temporal redundancy. To perform motion compensation in frame mode, the MPEG-2 video decoder should have a capacity to store two reconstructed frames (e.g., an anchor frame and the currently decoded frame) in the spatial domain. For decoding B-pictures, a third frame buffer is used to store a second anchor frame since B-pictures use a previously reconstructed and displayed frame and a reconstructed but not yet displayed frame as anchor frames for their decoding.
Because of the relatively large amount of data required to represent each HDTV frame, HDTV decoders must support higher data rates than SDTV decoders. The additional memory used by an HDTV decoder, as compared to a standard SDTV decoder, and the increased complexity associated with the inverse DCT circuit and other components of an HDTV decoder, can make an HDTV decoder considerably more expensive than an SDTV decoder.
In the conventional method of obtaining a low-resolution SD image sequence, the HD bitstream is fully decoded and then it is simply pre-filtered and sub-sampled. It is often referred to as a full-resolution decoder with spatial down-conversion. Although the quality is very good, the cost is typically quite high due to the large memory requirement.
In fact, the cost of memory alone may make an HDTV set incorporating an HDTV decoder prohibitively expensive for some consumers. It is expected that a fully MPEG compliant video decoder for HDTV may require a minimum of 10 MB of RAM for frame storage with a practical HDTV decoder probably requiring about 16 MB of relatively expensive Synchronous DRAM.
Therefore, it is desirable to provide a method and apparatus that permits one or more of the following advantages: (1) simplification of the complexity of the circuitry required to implement an HDTV decoder; (2) reduction in the amount of memory required to implement an HDTV decoder circuit; and (3) a single decoder that is capable of decoding both SDTV and HDTV signals. Furthermore, it is desirable that the cost of such a decoder be low enough that it is in a range that would be acceptable to most consumers, e.g., approximately the cost of an SDTV decoder.